Large public utility power plants produce electricity by coupling a generator to a mechanical power source such as a steam turbine. The generator comprises a rotor journaled in a stator, the rotor containing copper coil windings which carry a direct current for producing a magnetic flux. The power source spins the rotor at high speeds, causing the magnetic flux to sweep across copper coil windings in the stator and generate an electric current.
The generator stator comprises a cylindrical core having longitudinal slots along an inner surface, each slot containing a pair of top and bottom half turn coils. The half turn coils emerge from the longitudinal slots at each end of the stator core and loop in order to connect with other half turn coils thus forming a coil winding. The portion of a half turn coil which emerges from a slot for this connection is commonly referred to as a coil end turn. The plurality of end turns at each end of a stator core form an end winding basket. This basket arrangement of individual end turns must be consolidated into a unified structure in order to restrain radial and tangential coil movements which result from large electromagnetic forces, thermal expansion and coil vibration. Support rings and nonconductive braces are bolted through brackets to the stator core to brace the end turns and reduce coil wear which results from coil movement.
Efforts to reduce movement in the coil end windings of electric generators have resulted in several improved end turn support assemblies. For example, U.S. Pat. No. 4,563,607, issued Jan. 7, 1986 to Cooper, et al. and assigned to the assignee of the present invention, discloses a radial clamp assembly for clamping pairs of parallel top stator coil end turns with pairs of parallel bottom stator coil end turns. In these clamping arrangements, radial bands couple the top and bottom coil pairs in order to minimize movement of the individual end turns relative to the entire end winding basket. A top fiber glass support block is placed across the upper sides of two adjacent parallel top coil end turns, and a bottom fiber glass support block is placed across the lower sides of two adjacent parallel bottom coil end turns, with each of the support blocks having a central aperture. The two apertures are aligned with one another through a diamond shaped slot formed between the top and bottom end turn pairs. A resin impregnated fiber glass band comprising a number of turns of a fiber glass tape is passed through one of the apertures, and extends through each of the support block apertures. A pin is inserted through the band protruding through each support block to hold the band in position. The band is twisted to take up most of the slack. The band is then cured.
The curing process increases band tension strength from approximately 5,000 pounds to over 15,000 pounds. Curing is also believed to minimize the effects of moisture on the fiberglass. When curing is completed a tension device, such as a hydraulic jack, is used to pull at least one pin on each clamp assembly away from an adjacent support block in order to stretch the cured bands. Shims are then placed between the pins and the adjacent upper or lower blocks to keep the bands stretched after the pins are released. Spring washers or spring plates may also be positioned between the pins and support blocks to control loss of tension due to creep relaxation in the bands.
Such radial clamp assemblies have been installed on machines in the field as well as during factory assembly. However, the necessary curing process normally requires approximately four hours of continuous heating at 170.degree. C. In a typical installation having as many as 42 radial clamps per end winding basket, the bands must be individually cured by directing pairs of heat guns mounted on the top and bottom support blocks toward each band. Although several bands can be simultaneously cured, the installation process for forty two clamps may exceed twenty four hours. Precured bands have not been used in the past to reduce the installation time required for radial clamps because the cured bands cannot be twisted in place to remove excess slack. It is desirable to reduce the time required for band installation and in particular to develop a clamping configuration which allows for precuring of the glass fiber bands in order to expedite the installation process.
Recent improvements in stator end turn coil bracing systems include the positioning of a non-metallic segmented ring against the upper surfaces of the top coil end turns and the positioning of a non-metallic cone shaped support ring against the lower surfaces of the bottom coil end turns to provide a continuous rigid support about the end winding basket. These ring structures are used in lieu of top and bottom support blocks in some generators. The rings are formed of a semi-rigid baked fiber glass/epoxy resin composite. In order to provide sufficient mass and stiffness to reduce steady-state coil vibration and other sources of coil movement to acceptable levels, the structures must be relatively large. As a result there are very limited clearances about the end winding basket. Therefore, the aforedescribed process of installing uncured radial bands cannot be used to clamp the portions of coil end turns which are most subject to movement. It is therefore desirable to have an alternate band arrangement and method of clamp installation which provides for maximum radial clamping of top and bottom coils between the support ring structures of the type used in such generators.